The present invention relates to a lead frame for a resin-molded semiconductor device such as a multi-chip, power device serving as a control device applied to, for example, a power circuit (switching power supply).
In a case of a control device applied to a power circuit as an example, structures of a single-end type resin-molded semiconductor device and a conventional lead frame used for assembling the semiconductor device will be, described with reference to FIGS. 2(a) to 2(c). FIG. 2(a) is a plan view showing the lead frame, FIG. 2(b) is a view showing an assembled state that semiconductor elements are mounted and wire-bonded on the lead frame, and FIG. 2(c) is a plan view showing an appearance of the semiconductor device with leads being cut after resin-molding. In FIGS. 2(a) to 2(c), a reference numeral 1 denotes a lead frame; reference numerals 2 and 3 are power elements (switching elements) mounted on the lead frame 1; a reference numeral 4 is a control IC for the power elements 2 and 3; a reference numeral 5 is a bonding wire for internal wiring; and a reference numeral 6 is a resin package formed by resin-molding for surrounding the power element 2, the control IC 4, and the bonding wires 5.
In the conventional lead frame 1 shown in FIG. 2(a), a side rail 1a and a side rail 1b are disposed to extend at both sides of the lead frame 1 as a tie bar for transporting the lead frame 1 pitch by pitch. In between the side rails, die pads 7, 8 and 9 having all of leads arranged at one side with a regular interval; terminal leads 10 of a main circuit, control power supply, and signal circuit, separated from the die pads and extended to a side of the side rail 1b (numbers 1, 2, . . . assigned to the respective terminal leads 10 are terminal numbers), and a dam bar 11 connecting each of the terminal leads 10, are formed in a pattern in accordance with an outer dimension of the semiconductor device. It should be noted that the side rail 1a is connected to the die pad 7 via a tie bar 1a-1.
In a process of assembling the semiconductor device, the lead frame 1 is transported sequentially between steps. In the first step, chips of the power semiconductor elements 2 and 3 and the control IC 4 are mounted (die-bonded) on the die pads 7, 8, and 9 of the lead frame 1. In the next wiring step, the wires 5 are bonded between the terminal leads 10 and the power semiconductor elements 2, 3 and the control IC 4 for internal wiring. In the next molding (transfer molding) step, the lead frame 1 is set in a molding die (not shown), and a portion inside a mold line indicated by a hidden line in FIG. 2(b) is sealed with a resin to mold the resin package 6. Then, the side rails 1a and 1b and the dam bar 11 are cut to complete the semiconductor device shown in FIG. 2(c).
In the resin-molded semiconductor device assembled using the conventional lead frame shown in FIG. 2(a), however, there are problems in terms of the assembly and a cost as described below.
In a series of the steps performed on the lead frame for assembling the semiconductor device such as die bonding, wire bonding, and resin molding, the two side rails 1a and 1b extending at both sides support the patterned region of the conventional lead frame in order to ensure a strength enough for preventing the die pads patterned on the lead frame 1 and the wire-bonded terminal pads from warping or displacing while the lead frame is transported sequentially as shown in FIG. 2(a).
Thus, an additional step is required for cutting the tie bar after the resin molding, and an amount of pieces cut from the lead frame in the step (i.e. pieces such as the side rails and the dam bar) increases, thereby increasing a material cost of the lead frame. Moreover, in the completed state that the tie bar 1a-1 connecting the side rail 1a and the die pad 7 is cut-off after the resin-molding, as shown in FIG. 2(c), a remained part of the tie bar 1a-1 after cutting protrudes from an outside of the resin package 6 at a side opposite to the terminal leads 10, thereby exposing inner potential. Therefore, it is necessary to insulate the protruding part in an after-treatment step.
Further, when upgrading a version of the semiconductor device for adding a new function such as a protective function, the number of the terminals extended outside is increased according to a change in the circuit arrangement of the semiconductor device. Therefore, to manufacture the new version of the semiconductor device, the pattern of the lead frame is required to be changed, and the number of terminal leads led from the package to the outside is required to be increased. Therefore, it is necessary to produce a new molding die for the resin-molding in accordance with the increased number of the terminal leads, thereby increasing an investment for additional production equipment and a price of the semiconductor device.
It is therefore an object of the present invention to provide an improved lead frame for a resin-molded semiconductor device, which can reduce a material cost of the lead frame.
Another object of the invention is to provide a resin-molded semiconductor device as stated above, which can reduce the investment for additional production equipment in the case that the version of the semiconductor device is upgraded by adding a new function, thus improving the cost performance.
Further objects and advantages of the invention will be apparent from the following description of the invention.
To accomplish the above objects, according to the first aspect of the invention, a lead frame for a resin-molded semiconductor device is provided with die pads for separately mounting chips of power elements and a control IC thereon; terminal leads arranged in a row at one side and drawn from the die pads, and a main circuit, a control power supply and signal circuit separated from the die pads; and a dam bar connecting the terminal leads. In at least the power element, the main circuit terminals formed on the lead frame corresponding to the control IC, and the power elements, a plurality of leads is formed and drawn in advance. The power elements and the control IC mounted on the die pad are internally wired. After the lead frame is set in a mold die and is resin-molded, certain leads that are not used as the terminal leads are selected among the plurality of the leads, and the certain leads are cut and removed along with the dam bar.
As mentioned above, the number of the leads formed on the lead frame in advance is larger than the number of the terminals required for the semiconductor device. Thus, each lead serves as a beam to increase the mechanical strength (flexural rigidity) of the lead frame. Accordingly, it is possible to effectively eliminate warping or displacing of the lead frame during a series of steps of assembling the semiconductor device such as die bonding, wire bonding, and resin-molding performed on the lead frame.
Further, in a case that a version of the semiconductor device is upgraded by adding a protective function or the like, it is possible to cope with the upgraded version by changing only a part of the pattern of the inner leads and the die pads without modifying the arrangement of the outer leads relative to a basic pattern of the lead frame. Therefore, a resin package can be molded by using the existing molding die as it is without a new molding die.
Further, according to the second aspect of the invention, in addition to the configuration described above, a side rail of the lead frame is formed at only one side, and is connected to ends of the terminal leads. Specifically, the strength of the lead frame itself is improved by increasing the number of the leads. Thus, even though one side rail at the die pad side is omitted, the lead frame can be transported sequentially without any trouble as the side rail at one side connected to the ends of the terminal leads supports the die pads and the terminal leads. Since the side rail at one side is omitted, it is possible to reduce a material cost of the lead frame and the number of the steps required for cutting the tie bar after the resin-molding. Further, it is possible to eliminate a step of insulating because no part of the tie bar remains or is exposed from the outer surface of the resin package after cutting the tie bar.
Further, according to the third aspect of the invention, in addition to the configuration described above, dummy leads are formed along the terminal leads of the main circuit terminals with different potentials. The dummy leads are cut and removed along with the dam bar after the resin-molding. In the fourth aspect of the invention, inner ends of the dummy leads are cut along a mold line of a resin package.
With this arrangement, in the completed semiconductor device, a sufficient distance for insulation is formed between the terminal leads arranged at both sides with a space where the dummy lead has been removed, thereby ensuring a dielectric strength enough to withstand a main circuit voltage.
Further, in this case, the inner ends of the dummy leads may be extended up to the mold line of the resin package. Accordingly, the inner ends of the dummy leads prevent leakage of the resin from a molding die in the resin-molding step in which the lead frame is set in the molding die. Further, the dummy leads can be removed by cutting the dam bar because the inner ends of the dummy leads are not connected to the resin package after the resin-molding step.